For this circuit, the operating point of the cathode follower is set with the bias pot, R13. This sets the voltage at the grid of the cathode follower. The voltage on the anode is set by a source follower in my case, but it could as well have been a constant DC source of roughly 85 V. The load resistor, R10, will cause some current to be drawn through the tube under this condition. The tube will settle at some Vgk determined by the characteristics of the tube. In case of the ECC99, I seem to recall this being around Vgk = 5 V for the conditions noted in the schematic.

In summary, the voltage across the load resistor, R10, determines the current through the cathode follower. Ia = V(R10)/R10. If V(R10) is known, so is Vak of the cathode follower. From this, Vgk can be found on the plate curves.

So that's the DC operating point.

When a signal is applied, the cathode voltage will track the grid voltage. As you say, the gain is slightly less than unity, so a 1 V increase in grid voltage will cause an increase in cathode voltage of approximately 1 V. This in turn increases the anode current as V(R10) is now 1 V higher. You can work out the resulting change in Vgk from the plate curves. You'll find that it swings much less than the 1 V applied.

More progress. Here's the amp board. Measures 4.0 x 6.4 inches. The 300Bs go in the big holes (1.15" diameter; 4.5" center-to-center). I'll poke at it for another day or so before sending it out for fab.

The gain of my power amp is about 2 V/V (6 dB). So yeah... On the low side. Part of that is by design, actually. Running higher signal levels between the pre and power amp gives you better signal integrity as you aren't attenuating as much in the volume control only to gain it back up in the power amp.

To drive this amp to clipping you'll need a bit over 6 V RMS. So it definitely requires a preamp with some stones. I'm considering following it up with a preamp design, actually.

If you don't need differential input, you can take the input transformer out. I have options for a regular AC coupled input on the board. This will increase the power amp gain to 9 dB, hence, only requiring 4.25 V RMS to get to clipping.

That said, I use speakers that are roughly 90 dB efficient and have no trouble playing music loud. I typically run with the volume control set to -35 dB for background listening, -25 dB for actual listening to the music, and between -10 and 0 dB for loud. I'm driving my preamp with either a CD player - or these days more commonly an Airport Express through a Saffire DAC.

The board allows for selection of the following mains voltages: 120 V, 230 V, 240 V by moving a couple of wire jumpers. A soft-start circuit ensures that the inrush current is kept under control during start-up.

The output voltages are: 525 V (unregulated) for B+; -220 V (zener regulated) for bias; 2 x 20 V (unregulated) for filaments. In addition, an output for a POWER ON indicator LED is provided.

Now, of course 525 V and 20 V are too high for use directly. However, they're just about perfect for use with my 21st Century Maida Regulator for generating 400 V B+ and my Universal Filament Regulators for generating 6.3 V and 5.0 V. I run the two 5.0 V regulators from one 20 V output and the regulator for the 6.3 V off of its own 20 V output. Works great.

For those who wish to use lower B+ voltages, two wires can be swapped on the transformer secondary, resulting in an unregulated voltage of roughly 450 V. This would be quite suitable for regulated B+ voltages in the 340~380 V range.

And now for pictures of the amplifier board. Note that the tube sockets are mounted on the back side of the board such that the tubes can poke through the top plate of the chassis. The large cutouts are where the 300B sockets go. The sockets will be mounted to the top plate of the chassis and their pins will poke through the cutouts in the PCB. The pins will be connected by short lengths of wire to holes in the PCB.
In addition to the sockets, I've chosen to mount the bias pots on the back side of the board as well. That way, I'll be able to adjust the bias through a hole in the top plate of the chassis. You can mount the pots on the component side of the board if you prefer adjustment from the bottom of the amp.

All holes for components or screws that will poke through or require access through the top chassis panel are laid out symmetrically. This was actually a bit of a challenge, but I think it'll result in a really nice looking top plate on the chassis.

The 300Bs are 4.5 inches apart. The two driver tubes are two inches apart and centered between the 300Bs. It's a pretty neat and compact layout.

The board allows for use of a Jensen JT-11P-1HPC input transformer. If you would prefer a single-ended input, the board supports the use of an input capacitor instead.

The board has direct support for 6N6P (as pictured), ECC99, and 12BH7A. Switching between tube types require swapping a couple of components and wire links. The board may also be used with d3A tubes, though, this will require some creative wiring to connect two d3A pentodes to a socket that's intended for use with a dual triode. I've done it with good results, though.